Germ cells are cells that are specialized to produce haploid gametes in multicellular organisms. Germ cell tumors represent a diverse family of neoplasms affecting a wide range of patients. The great majority of testicular tumors (the most common malignancy in young men) are malignant germ cell tumors. The most common ovarian tumor, the benign or “mature” teratoma, is also of germ cell origin. Additionally, malignant ovarian germ cell tumors and germ cell tumors of various subtypes are of relatively common occurrence in children. Interestingly, benign and malignant germ cell tumors also arise in extragonadal locations (the mediastinum and central nervous system). Although the histogenesis of these extragonadal tumors is poorly understood, they exhibit similar biological behavior to their gonadal counterparts.
Malignant germ cell tumors are subdivided based on histologic appearance into pure seminomas (most common), and nonseminomas that include embryonal carcinoma, teratoma, choriocarcinoma, and yolk sac tumor. Patterns of mixed histologic appearance are also of common occurrence. Seminoma closely resembles primitive germ cells without evidence of further differentiation. Embryonal carcinoma is more anaplastic than seminoma, often with gland-like areas, and is believed to represent early differentiation to other lineages. The remaining histologic subtypes recapitulate the various lineages of the primitive embryo, reflecting the totipotent character of germ cells. Teratoma, choriocarcinoma, and yolk sac tumors exhibit evidence of fetal, placental, and yolk sac differentiation, respectively.
Accurate tissue-based diagnosis and subtyping of germ cell tumors (i.e., seminomas vs. nonseminoma) is of paramount importance for the patient, due to differences in staging evaluation and subsequent management (See, e.g., Ch. 98 on “Testicular Cancer” in Harrison's Principles of Internal Medicine, 14th Edition, 1997, McGraw-Hill, Inc., N.Y.—hereinafter “Harrison's”). Yet, some germ cell tumors are difficult or even impossible to reliably diagnose or subtype due to the diverse histologic appearances of germ cell tumors and the existence of numerous other malignancies that can mimic germ cell tumors histologically. For example, clear cell carcinoma of the ovary can histologically resemble dysgerminoma (the ovarian counterparts of seminoma); mediastinal thymomas can be difficult to distinguish from germ cell tumors, and testicular lymphoma can mimic seminoma (Scully et al, 1999, Armed Forces Institute of Pathology, Washington, D.C.; Suster et al., Seminars in Diagnostic Pathology, 1995, 12(1): 98-104). In addition, metastases in patients with unsuspected primary germ cell tumors can be initially misdiagnosed, resulting in treatment delays.
Tumor-specific markers have been clinically useful for a variety of reasons, including accurate tissue-based diagnosis by immunohistochemistry, population-based screening, confirmation of a clinical diagnosis prior to surgery, and monitoring of patients in remission. In general, the clinical utility of a marker is directly proportional to its specificity. For example, β-hCG, a highly sensitive and specific marker for trophoblast, is indispensable in the diagnosis and clinical monitoring of patients with choriocarcinoma (it is also the basis of pregnancy tests), and α-fetoprotein (AFP) serves as a fairly sensitive marker of yolk sac differentiation. Serum assays for both AFP and β-hCG are routinely employed in the diagnosis of patients with suspected germ cell tumors to determine if non-seminomatous components are present (which would alter patient management) (Harrison's). Commercially-available AFP and β-hCG antibodies are also routinely employed in immunohistochemical assays performed by hospital laboratories on paraffin-embedded formalin-fixed tissue.
Currently available immunohistochemical markers for seminoma are relatively non-specific and no useful serum seminoma tumor markers exist, even though seminoma is the most common germ cell tumor subtype. Although placental-type alkaline phosphatase (PLAP) is a fairly sensitive (˜80%) marker of seminoma, it is far from specific, being expressed in a variety of carcinomas including the majority of ovarian carcinomas, a significant number of gastrointestinal carcinomas, and several normal tissues (Sunderland et al, Cancer Research, 1984, 44(10): 4496-4502). Largely because of this lack of specificity, alkaline phosphatase serum assays are not being utilized routinely. Furthermore, PLAP is not reliably expressed in normal germ cells (Perry et al., Human Pathology, 1994, 25(3): 235-239). The development of a more specific and sensitive marker of seminoma would represent a major advance.
The vasa gene was originally described in Drosophila, where various studies including whole-mount in situ staining have, reportedly, shown that vasa is expressed only in germ cells (of both sexes) at all stages of development, from the preblastoderm stage to primitive germ cells to gametogenesis in adults. A number of investigations have, reportedly, shown that vasa is not only germ-cell specific in its expression, it is absolutely required for germ cell development, and in Drosophila, vasa mutants fail to develop germ cells (Lasko et al., Nature, 1988, 335:611-617, and SEQ ID NO:7). Subsequently, vasa homologues were identified in mouse, zebrafish, and Xenopus. In all of the foregoing species, expression was reportedly germ-cell specific and occurred throughout life. Reports on vasa expression in these organisms have greatly increased our knowledge of the germ-cell lineage, allowing germ-cell lineage cells to be traced back to the 4-cell morula stage (Fujiwara, et al., Proc. Nat. Acad Scie. USA, 1994, 6;91(25): 12258-12262; Yoon, et al., Development, 1997, 124:3157-3166; Ikenishi, et al., Dev. Grow., and Diff, 1997, 39:625-633).
There exists a need to identify agents that are useful in the diagnosis of tumors of germ cell origin.
These and other objects will be described in greater detail below.